The notion of interaction is essential to human-computer interaction, yet rarely studied. We use quantitative and qualitative methods to investigate how this notion has been used across 35 years of proceedings from the ACM Conference on Human Factors in Computing (CHI). Using natural language processing, we extract 53,568 occurrences of the word “interaction” across 4,604 papers. In these occurrences, we categorize 2,668 unique words that modify how “interaction” is used in a sentence. We show that the use of “interaction” is both increasing and diversifying, suggesting the importance of the notion, but also the difficulty in developing theory about interaction. Our findings show that styles of interaction are closely associated with changes in technology and that modalities and characteristics of interaction are becoming more of a topic than specific devices or widgets. Interaction qualities, relating to structure, feel, effectiveness, and efficiency, are consistently prominent, and the quality of novelty is increasingly frequent. From this analysis, we identify open questions about interaction, including how to build knowledge across changing technologies, how to work toward a model of quality for interaction, and what the core of a science of interaction could be.